The invention relates to a differential absorption polarizer more particularly for forming an integrated optics polarizer. It also relates to the method of forming such a polarizer and an apparatus implementing this method.
Several techniques have been proposed and experimentally verified for forming an integrated optics polarizer element. In brief, they may be grouped into two categories :
Elimination of one of the transverse electric TE or transverse magnetic TM polarizations by differential absorption ;
Spacial separation of the two polarizations, one remaining guided and the other being placed at the cut-off.
In general, the second type of polarizer leads to complex configurations some of which are not always compatible with the planar geometry of the integrated optics. Thus, there may be mentioned as example the use at the surface of the waveguide of an anisotropic crystal one of whose refraction indexes would be, for one of the optical polarizations higher than the refraction index of the waveguide. A description of such a system will be found in the article "ANISOTROPIC POLARIZERS FOR Ti:LiNb03 STRIP WAVEGUIDES" by M. PAPUCHON et al published in the Technical Digest on the 24th-26th Apr. 1984 (KISSIMMEE, Fla. 7th topical meeting INTEGRATED AND GUIDED-WAVE OPTICS-WC5). In other cases, the anisotropic properties of the substrate itself may be used (eg : LiN03) for obtaining the desired effect in situ. It can in fact be shown that, in the case of LiNb03, if the optical axis is in the plane of the substrate for some angles, of the waveguides with this optical axis, the propagation of "quasi TE" waves takes place with the losses which may be very high (&gt;50dB over 10 mm demonstrated in the laboratory). This configuration may form a disadvantage during integration of other functions with particular characteristics.
The invention uses then the elimination of one of the transverse magnetic TM or transverse electric TE polarizations by differential absorption.
Polarizers using this effect are known in the technique.
In these polarizers, polarization effect is obtained by creating a differential absorption between the quasi TE and quasi TM polarized modes because of the high attenuation which may be induced in the TM wave using a metal layer. To increase the efficiency of the interaction, a dielectric buffer layer must be inserted between the metal and the waveguide. The optogeometric parameters of this layer generally depend critically on the optical constants of the metal used. This is the case in particular for the thickness of the dielectric buffer layer which must be defined with high accuracy (about 100 .ANG.). The article by K. THYAGARAJAN et al having for title "EXPERIMENTAL DEMONSTRATION OF TM MODE-ATTENUATION RESONANCE IN PLANAR METAL-CLAD OPTICAL WAVEGUIDES" published in Optics Letters volume 10, number 6, in June 1985, pages 288 to 290, demonstrates the importance of the thickness of the dielectric buffer layer for obtaining efficient polarization. This particular thickness is in general difficult to obtain.
To try to overcome this drawback, a known arrangement consists in bevelling the dielectric buffer layer as is described in the article "EXPERIMENT STUDIES OF METAL-CLAD TAPERED OPTICAL WAVEGUIDES" by C. NGUYEN et al published in Electronic Letters of the 24th of May 1984, volume 20, number 11, p. 439.
Thus, the mode to be absorbed, will find in the bevelled thickness of the dielectric layer a thickness adapted to better absorption.
However, such a bevelled structure is difficult to obtain and, in addition, does not always allow a good efficiency to be obtained.
This is why the invention provides a polarizer easy to construct and allowing efficient polarization to be obtained.